Scientists have recognized that life can
be organized into several different levels of function
and complexity. These functional levels are: species, populations, communities,
and ecosystems.
Species
Species are
the different kinds of organisms found on the Earth.
A more exact definition of species is a
group of interbreeding organisms that do not ordinarily
breed with members of other groups. If a species
interbreeds freely with other species, it would no longer
be a distinctive kind of organism. This definition works
well with animals. However, in some plant species fertile
crossings can take place among morphologically and physiologically
different kinds of vegetation. In this situation, the
definition of species given here is not appropriate.
Populations
A population comprises
all the individuals of a given species in a specific
area or region at a certain time. Its significance is
more than that of a number of individuals because not
all individuals are identical. Populations contain genetic
variation within themselves and between other
populations. Even fundamental genetic characteristics
such as hair color or size may differ slightly from individual
to individual. More importantly, not all members of the
population are equal in their ability to survive and
reproduce.
Communities
Community refers
to all the populations in a specific area or region at
a certain time. Its structure involves many types of interactions among
species. Some of these involve the acquisition and use
of food, space, or other environmental resources. Others
involve nutrient cycling through all members of the community
and mutual regulation of population sizes. In all of
these cases, the structured interactions of populations
lead to situations in which individuals are thrown into
life or death struggles.
In general, ecologists believe that a community that
has a high diversity is
more complex and stable than
a community that has a low diversity. This theory is
founded on the observation that the food
webs of communities of high diversity are more
interconnected. Greater interconnectivity causes these
systems to be more resilient to disturbance.
If a species is removed, those species that relied on
it for food have the option to switch to many other species
that occupy a similar role in that ecosystem. In a low
diversity ecosystem, possible substitutes for food may
be non-existent or limited in abundance.
Ecosystems
Ecosystems are
dynamic entities composed of the biological community and
the abiotic environment.
An ecosystem's abiotic and biotic composition and structure
is determined by the state of a number of interrelated
environmental factors. Changes in any of these factors
(for example: nutrient availability, temperature, light
intensity, grazing intensity, and species population
density) will result in dynamic changes to the nature
of these systems. For example, a fire in the temperate
deciduous forest completely changes the structure of
that system. There are no longer any large trees, most
of the mosses, herbs, and shrubs that occupy the forest
floor are gone, and the nutrients that were stored in
the biomass are quickly released into the soil, atmosphere
and hydrologic system. After a short time of recovery,
the community that was once large mature trees now becomes
a community of grasses, herbaceous species, and tree
seedlings.